In cellular communication systems, there is a trend of enabling/realizing interworking between a cellular radio access network (RAN), such as a LTE, LTE-A or UMTS radio access network (e.g. E-UTRAN), and a wireless area network (WLAN) so as to enhance capacity and/or coverage. Such interworking includes radio level integration/aggregation hereinafter denoted as RAN-WLAN radio aggregation. The RAN-WLAN radio aggregation basically provides for dual connectivity for terminals being simultaneously served by both the RAN and the WLAN via respective radio interfaces, wherein the serving RAN network element (e.g. eNodeB or RNC) and the serving WLAN network element (e.g. WLAN AP or WAG) are interconnected, while being either collocated, i.e. implemented in an integrated manner, or non-collocated, i.e. implemented in a separate manner with a near-ideal backhaul link there-between.
Conceptually, such RAN-WLAN radio aggregation would be alike LTE dual connectivity with bearer split functionality (typically referred to as DC-3C) currently under standardization in 3GPP. Alike the latter, in RAN-WLAN radio aggregation, the RAN network element would act as the master node (or, stated in other words, the anchor point), and the WLAN network element would act as the slave node. The main objectives in this regard are the support of network-controlled mechanisms which enable spectrum aggregation gains (including fine load balancing between RAN and WLAN).
In order to support RAN-WLAN radio aggregation, a specified (logical) interface between the RAN network element and the WLAN network element is required for facilitating user-plane data forwarding from the RAN over the WLAN to dual-connectivity terminals, such that proper combination of user-plane data via the RAN path and the WLAN path is enabled at the terminal side, under the assumption that the anchor point of the aggregation is located at the RAN network element.
More specifically, when performing packet-wise radio level integration/aggregation between RAN and WLAN, the RAN network element (e.g. eNodeB or RNC) determines on a packet basis whether to transmit a packet over the RAN path or the WLAN path to the terminal. The determination can be based on various performance metrics related to the dynamic performance of the two networks in terms of e.g. available capacity, packet delay, packet loss rate per network, or the like.
In order to optimally adjust the user-plane data flow (i.e. increase/maintain/decrease the data flow speed) between the RAN network element (e.g. eNodeB or RNC) and the WLAN network element (e.g. WLAN AP or WAG), a flow control mechanism is required. To this end, a feedback of performance metrics from the WLAN to the RAN is required as a basis for flow control.
In order to achieve good performance for dual connectivity and make aggregation work in practice, reliable/assured user-plane data packet delivery is required. Otherwise, unreliable/unassured packet delivery including packet losses would be harmful to the aggregation in stalling higher layer protocols such as TCP which would interpret e.g. packet losses as an ongoing congestion slowing down the data rate.
Accordingly, there is a demand for enabling/realizing an interface functionality for RAN-WLAN radio aggregation, i.e. radio level integration/aggregation between a cellular radio access network and a wireless area network.